U.S. patent number 8,208,939 [Application Number 12/025,184] was granted by the patent office on 2012-06-26 for dual bandwidth time difference of arrival (tdoa) system.
This patent grant is currently assigned to Aeroscout Ltd.. Invention is credited to Daniel Aljadeff, Reuven Amsalem, Amir Lavi, Adi Shamir.
United States Patent |
8,208,939 |
Aljadeff , et al. |
June 26, 2012 |
**Please see images for:
( Certificate of Correction ) ** |
Dual bandwidth time difference of arrival (TDOA) system
Abstract
A wireless location system has at least one wireless tag to be
located by the wireless location system, wherein the at least one
wireless tag transmits two wireless signals having a known time
relationship and having different bandwidths. A plurality of
receivers is provided wherein a first receiver receives and
processes a first of the two wireless signals and estimates a time
to arrive (TOA) of the first wireless signal, and a second receiver
receives and processes a second of the two wireless signals and
estimates a TOA of the second wireless signal at the second
transceiver. The plurality of receivers is time synchronized based
on a common timing signal. A location server is coupled to each of
the plurality of receivers. The location server receives the TOA of
the first wireless signal from the first receiver and the TOA of
the second wireless signal from the second receiver. The location
server calculating a TDOA of the two wireless signals and estimates
a position of the at least one wireless tag based on the TDOA.
Inventors: |
Aljadeff; Daniel (Kiriat Ono,
IL), Amsalem; Reuven (Nes-Ziona, IL), Lavi;
Amir (Rehovot, IL), Shamir; Adi (Kidron,
IL) |
Assignee: |
Aeroscout Ltd. (Park
Tamar--Rehovat, IL)
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Family
ID: |
39675719 |
Appl.
No.: |
12/025,184 |
Filed: |
February 4, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080186231 A1 |
Aug 7, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60888220 |
Feb 5, 2007 |
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Current U.S.
Class: |
455/456.1;
455/456.2; 455/456.3 |
Current CPC
Class: |
G01S
5/0257 (20130101); G01S 5/06 (20130101) |
Current International
Class: |
H04W
24/00 (20090101) |
Field of
Search: |
;455/456.1,456.2,456.3,404.1,404.2,456.5,502,507,552.1
;342/457,357.72,450,357.16 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
European Search Report for EP08737608, mailed Apr. 27, 2011 (10
pages). cited by other.
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Primary Examiner: Gelin; Jean
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner LLP
Parent Case Text
RELATED PATENT APPLICATIONS
The present application is related to and claims the benefits of
U.S. Provisional Application entitled "DUAL BANDWIDTH TDOA SYSTEM",
filed Feb. 5, 2007, having a Ser. No. 60/888,220 and in the name of
the same inventors listed above.
The present application is also related to U.S. patent application
entitled "METHOD AND SYSTEM FOR LOCATION FINDING IN A WIRELESS
LOCAL AREA NETWORK", filed on Aug. 20, 2002, having a Ser. No.
10/225,267; U.S. Pat. No. 6,968,194, entitled "METHOD AND SYSTEM
FOR SYNCHRONIZING LOCATION FINDING MEASUREMENTS IN A WIRELESS LOCAL
AREA NETWORK", issued on Nov. 22, 2005; and U.S. Pat. No.
6,963,289, entitled "WIRELESS LOCAL AREA NETWORK (WLAN) CHANNEL
RADIO-FREQUENCY IDENTIFICATION (RFID) TAG SYSTEM AND METHOD
THEREFOR", issued on Nov. 8, 2005; the specifications of which are
herein incorporated by reference.
Claims
What is claimed is:
1. A wireless location system comprising: at least one wireless
transmitter unit to be located by said wireless location system,
wherein the at least one wireless transmitter unit transmits two
wireless signals having a known time relationship and having
different bandwidths, wherein the two wireless signals comprise an
ultra wide band (UWB) signal and a narrower band signal, and
wherein the UWB signal and the narrower band signal are both
transmitted with a known time relationship between the UWB signal
and the narrower band signal; a plurality of receivers, wherein a
first receiver receives and processes a first of the two wireless
signals and estimates a time to arrive (TOA) of the first wireless
signal, and a second receiver receives and processes a second of
the two wireless signals and estimates a TOA of the second wireless
signal at the second receiver, said second receiver selecting an
earliest TOA of said received signals, wherein the plurality of
receivers is time synchronized based on a common timing signal, and
wherein at least one receiver of the plurality of receivers
comprises: a component that receives and processes each of said two
wireless signals; a component that estimates a TOA of each of said
two wireless signals at said at least one receiver, wherein said
TOAs estimated by the at least one receiver is referenced to
synchronized time counters; and a location server coupled to each
of the plurality of receivers, the location server receiving the
TOA of the first wireless signal from the first receiver and the
selected TOA of the second wireless signal from the second
receiver, the location server calculating a time difference of
arrival (TDOA) of the two wireless signals and estimating a
position of the at least one wireless transmitter unit based on the
TDOA.
2. A wireless location system unit according to claim 1, wherein
the UWB signal carries no data and the narrower band signal carries
transmitter data.
3. A wireless location system in accordance with claim 1, wherein
the location server estimates a position of the at least one
wireless transmitter unit based on multiple TDOA calculations.
4. A wireless location system according to claim 3, wherein the
common timing signal to time synchronize the plurality of receivers
is a wireless signal.
5. A wireless location system in accordance with claim 1, wherein
the TDOA comprises at least one TDOA value calculated from a
difference of one TOA estimated on the transmitted UWB signal and a
second TOA estimated on the transmitted narrower band signal.
6. A wireless location system according to claim 1, wherein at
least one receiver is a transceiver and transmits the common timing
signal.
7. A wireless location system according to claim 1, wherein at
least one wireless tag at a known position transmits the common
timing signal used to time synchronize the plurality of
receivers.
8. A wireless location system in accordance with claim 1, wherein
said TOA of each of said two wireless signals is referenced to a
single time counter.
9. A wireless location system comprising: at least one wireless
transmitter unit to be located by said wireless location system,
wherein the at least one wireless transmitter unit transmits two
wireless signals having a known time relationship and having
different bandwidths; a plurality of receivers, wherein a first
receiver receives and processes a first of the two wireless signals
and estimates a time to arrive (TOA) of the first wireless signal,
and a second receiver receives and processes a second of the two
wireless signals and estimates a TOA of the second wireless signal
at the second receiver, wherein the plurality of receivers is time
synchronized based on a common timing signal; and a location server
coupled to each of the plurality of receivers, the location server
receiving the TOA of the first wireless signal from the first
receiver and the TOA of the second wireless signal from the second
receiver, the location server calculating a time difference of
arrival (TDOA) of the two wireless signals and estimating a
position of the at least one wireless transmitter unit based on the
TDOA, wherein the location server assigns a quality factor to each
TOA reported by the plurality of receivers, and wherein the
estimated position of the wireless transmitter unit is based on the
assigned quality factors.
10. A wireless location system in accordance with claim 1, wherein
said at least one wireless transmitter unit is a wireless tag
comprising: a first signal transmitter having a first signal
antenna; a second signal transmitter having a second signal
antenna; and a controller coupled to the first signal transmitter
and the second signal transmitter.
11. A wireless location system in accordance with claim 10, wherein
the wireless tag further comprises: a second signal receiver; and a
switching unit coupled to the second signal transmitter and the
second signal receiver, wherein the controller is coupled to the
first signal transmitter, the second signal transmitter, and the
second signal receiver.
12. A wireless location system in accordance with claim 11, wherein
the second signal antenna is coupled to the switching unit.
13. A wireless location system comprising: at least one wireless
transmitter unit to be located by said wireless location system,
wherein the at least one wireless transmitter unit transmits two
wireless signals having a known time relationship and having
different bandwidths; a plurality of receivers, wherein a first
receiver receives and processes a first of the two wireless signals
and estimates a time to arrive (TOA) of the first wireless signal,
and a second receiver receives and processes a second of the two
wireless signals and estimates a TOA of the second wireless signal
at the second receiver, wherein the plurality of receivers is time
synchronized based on a common timing signal, wherein the TOA of
the first and second signals are referenced to synchronized time
counters, and wherein at least one of the plurality of receivers
comprises: a first signal receiver; a second signal receiver; a
first TOA circuit coupled to the first signal receiver to calculate
the TOA of the first signal; and a controller coupled to the first
TOA circuit, the second signal receiver, and means for processing
the second signal, the controller calculating the TOA of the second
signal; and a location server coupled to each of the plurality of
receivers, the location server receiving the TOA of the first
wireless signal from the first receiver and the TOA of the second
wireless signal from the second receiver, the location server
calculating a time difference of arrival (TDOA) of the two wireless
signals and estimating a position of the at least one wireless
transmitter unit based on the TDOA.
14. A wireless location system comprising: at least one wireless
transmitter unit to be located by said wireless location system,
wherein the at least one wireless transmitter unit transmits two
wireless signals having a known time relationship and having
different bandwidths; a plurality of receivers, wherein a first
receiver receives and processes a first of the two wireless signals
and estimates a time to arrive (TOA) of the first wireless signal,
and a second receiver receives and processes a second of the two
wireless signals and estimates a TOA of the second wireless signal
at the second receiver, wherein the plurality of receivers is time
synchronized based on a common timing signal, wherein the TOA of
the first and second signals is referenced to synchronized time
counters, and wherein at least one of the plurality of receivers
are transceivers comprising: a first signal transceiver receiver; a
second signal transceiver transmitter; a second signal transceiver
receiver; a first TOA circuit coupled to the first signal
transceiver receiver to calculate the TOA of the first signal; a
controller coupled to the first TOA circuit, the second signal
transceiver transmitter, and the second signal transceiver
receiver, and means for processing the second signal, the
controller calculating the TOA of the second signal; a switching
unit coupled to the second signal transceiver transmitter, and the
second signal transceiver receiver; a first signal transceiver
antenna coupled to the first signal transceiver receiver; and a
second signal transceiver antenna coupled to the switching unit;
and a location server coupled to each of the plurality of
receivers, the location server receiving the TOA of the first
wireless signal from the first receiver and the TOA of the second
wireless signal from the second receiver, the location server
calculating a time difference of arrival (TDOA) of the two wireless
signals and estimating a position of the at least one wireless
transmitter unit based on the TDOA.
15. A method for estimating a time of arrival (TOA) of a wireless
transmission at a wireless receiver, comprising: transmitting an
ultra wide band (UWB) signal and a narrower band signal from a
single wireless device, the UWB signal and the narrower band signal
having a known time relationship; receiving the UWB signal and the
narrower band signal at a receiver unit, the receiver unit
processing the UWB signal and the narrower band signal and
estimating a TOA of the UWB signal and a TOA of the narrower band
signal at said receiver unit, said TOAs of said received signals
being referenced to synchronized time counters; and selecting an
earliest TOA between the TOA of the UWB signal and the TOA of the
narrower band signal.
16. A method according to claim 15, further comprising estimating a
quality of the estimated TOA values.
17. A method according to claim 16, further comprising correcting
the TOA of the narrower band signal by using the TOA of the UWB
signal.
18. A method according to claim 15, wherein the earliest TOA is
reported to a location server coupled to the receiver unit.
19. A method according to claim 18, wherein a TOA of a non-selected
signal is also reported by the receiver unit.
Description
FIELD OF INVENTION
The present invention relates generally to wireless networks, and
more specifically, to a method and system to improve location
accuracy while maintaining the advantages of an IEEE 802.11x based
location system.
BACKGROUND OF THE INVENTION
In many WLAN and other wireless data networks implementations, it
is beneficial for the system owner to know the physical location of
mobile clients or compatible tags. This will enable new features
such as enhanced network security, providing of `location based`
services, asset tracking and many others.
A typical `location finding` system, as currently implemented in
the related patents and patent applications disclosed above,
consists of multiple `location transceivers` connected to the WLAN
system, either by means of CAT-5 backbone or by wireless bridges.
The typical `location transceiver` contains a WLAN receiver and the
circuitry required to extract Time of Arrival (TOA) information and
report this information to the location server of the system. The
`location server` performs the required computation of the client
or tag location based on the known location of the location
transceivers, and displays it to the user or reports it to the
requesting application.
In an IEEE 802.11a/b/g/n based Time Difference of Arrival (TDOA)
location system, the TDOA of each pair of location transceivers is
calculated from the reported TOA's that are calculated on a single
802.11a/b/g/n transmitted message. In a wireless local area data
communication system, the Location Transceivers may be attached
and/or integrated and/or be a part of the Access Points in said
network.
The time synchronization of such a system using wireless or wired
methods has also been previously done. Regarding wireless
synchronization, U.S. Pat. No. 6,968,194 B2, entitled "METHOD AND
SYSTEM FOR SYNCHRONIZING LOCATION FINDING MEASUREMENTS IN A
WIRELESS LOCAL AREA NETWORK", describes a location system in which
multiple location receivers compute the time-of-arrival (TOA) of a
reference transmitter signal, which is generally a beacon signal.
The TOAs are collected and reported to a master unit that contains
stored predetermined position information for the location
receivers. The master unit computes the time-differences-of-arrival
(TDOA) between multiple receivers and computes differences between
the measured TDOAs and theoretical TDOAs computed in conformity
with the predetermined position of each location receiver. The
deviations between theoretical and measured TDOAs are collected in
a statistical sample set and Kalman filters are used to produce a
model of location receiver timebase offset and drift over multiple
received beacon signals. The filter outputs are used to then either
correct subsequent TDOA measurements for each location receiver,
improving the accuracy of subsequent and/or prior TDOA
measurements, or commands are sent to the location receivers to
calibrate the timebases within the location receivers in order to
improve the accuracy of subsequent TOA measurements.
The location accuracy of such a system is determined among many
other factors by the accuracy of the TOA as calculated by the
Location Transceivers and/or Access Points. The accuracy of the
TOA, especially in multipath environments, is strongly affected by
the bandwidth of the received signal and the limited bandwidth of
the IEEE 802.11a/b/g/n signals is a strong limiting factor in
achieving better location accuracy than 1-2 m.
In some location systems it is desirable to achieve an improved
location accuracy compared to the accuracy achieved by IEEE
802.11a/b/g/n systems. It's well known that the location accuracy
is strongly affected by the accuracy of the estimated TOA by each
of the location transceivers, and the TOA maximum accuracy is
mainly determined by the bandwidth of the received signal. Other
factors as the signal SNR, time synchronization of the
transceivers, modulation type, number of transceivers, etc. also
affect the location accuracy.
Ultra wide band (UWB) systems have been designed to allow digital
communication at very high data rates by using very wide spectrum
bands (typically more than 500 MHz). Since the use of this huge
band overlaps many other licensed and unlicensed bands, this
technology has been limited to very low average transmission power
(an average of less than -40 dBm/MHz) thus strongly limiting the
communication range.
However, the use of those extremely wide bands is beneficial for
time based location systems, since it allows a very accurate and
precise TOA measurement in addition to an excellent separation of
multipaths, even in the presence of very close multipaths (up to
20-30 nsec).
Therefore, a well designed UWB location system can achieve a
typical location accuracy of .+-.1 feet while in several cases it's
possible to achieve a location accuracy of few inches. However as
previously mentioned, the range of such location systems is
limited, in addition to other limitations as imposed by the
regulatory bodies.
In such a location system (either IEEE 802.11a/b/g/n or UWB), a tag
or standard client is required to transmit one or several messages
to allow all those location transceivers or AP's to receive and
measure the TOA of the transmitted messages. Those location
transceivers maybe time synchronized by cables and/or over the air.
The location transceivers calculate the TOA of each received
message and report those values to a server which calculates the
tag or mobile unit position. However, as stated above, both systems
have some disadvantages.
Therefore, it would be desirable to provide a method and system to
overcome the above problems. The system and method would provide a
combined (IEEE 802.11a/b/g/n and UWB) location system in which the
integration of both technologies provides a location system with
significant advantages not present on each of those technologies
separately.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, a
wireless location system is disclosed. A wireless location system
is disclosed. The wireless location system has at least one
wireless tag to be located by the wireless location system, wherein
the at least one wireless tag transmits two wireless signals having
a known time relationship and having different bandwidths. A
plurality of receivers is provided wherein a first receiver
receives and processes a first of the two wireless signals and
estimates a time to arrive (TOA) of the first wireless signal, and
a second receiver receives and processes a second of the two
wireless signals and estimates a TOA of the second wireless signal
at the second transceiver. The plurality of receivers is time
synchronized based on a common timing signal. A location server is
coupled to each of the plurality of receivers. The location server
receives the TOA of the first wireless signal from the first
receiver and the TOA of the second wireless signal from the second
receiver. The location server calculating a TDOA of the two
wireless signals and estimates a position of the at least one
wireless tag based on the TDOA
In accordance with another embodiment of the present invention, a
method for estimating a time to arrive of a wireless transmission
at a wireless receiver is disclosed. The method comprises:
transmitting an UWB signal and a narrower band signal from a single
wireless device, the UWB signal and a narrower band signal having a
known time relationship; receiving the UWB signal and the narrower
band signal at a receiver unit, the receiver unit processing the
UWB signal and a narrower band signal and estimates a time of
arrival (TOA) of the UWB signal and a TOA of the narrower band
signals; and selecting an earliest TOA between the TOA of the UWB
signal and the TOA of the narrower band signal.
In accordance with another embodiment of the present invention, a
method for calculating a time difference of arrival (TDOA) of a
wireless transmission at a plurality of wireless receivers is
disclosed. The method comprises: transmitting two wireless signals
with known time relationship between them having different
bandwidths by a single wireless device; receiving and processing a
first of the two wireless signals and estimating a time of arrival
(TOA) of a first of the two wireless signals by a first
transceiver, receiving and processing a second of the two wireless
signals and estimating a TOA of a second of the two wireless
signals by a second transceiver; transmitting the first TOA and
second TOA by the first and second transceivers to a common
location server; and calculating the TDOA of the two wireless
signals from a difference of the TOA of the first signal and the
second TOA of the second signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features believed characteristic of the invention are set
forth in the appended claims. The invention itself, as well as a
preferred mode of use, and advantages thereof, will best be
understood by reference to the following detailed description of
illustrated embodiments when read in conjunction with the
accompanying drawings, wherein like reference numerals and symbols
represent like elements.
FIG. 1 is a pictorial diagram depicting a wireless network
organized in accordance with an embodiment of the present
invention;
FIG. 2 is a diagram showing the combined tag message of a tag used
in the wireless network of the present invention;
FIG. 3 is a detailed timing diagram of a combined UWB+WLAN signal
used in the wireless network of the present invention;
FIG. 4 is a simplified block diagram of a combined UWB+WLAN tag
used in the wireless network of the present invention; and
FIG. 5 is a simplified block diagram of a location transceiver used
in the wireless network of the present invention.
FIG. 6 is a simplified block diagram showing the TOA counters
section comprised in the timing function of a location transceiver
used in the wireless network of the present invention.
DESCRIPTION OF THE INVENTION
The present invention describes a method, system and units of a
dual bandwidth TDOA location system 1. More specifically the
present invention applies to a TDOA location system 1 integrating
UWB technology with a narrower band WLAN/WPAN communication system.
It also applies to methods of TOA and TDOA calculation of dual
bandwidth transmissions. The present invention further describes a
wireless transmitter able to transmit a dual bandwidth signals and
a receiver able to receive and process such dual bandwidth
transmissions. In accordance with one embodiment of the present
invention, the WLAN system 1 will consist of an IEEE 802.11a/b/g
(i.e. anyone of IEEE 802.11a, IEEE 802.11b or IEEE 802.11g and the
like) but the same principles and ideas can be implemented using
other communication standards as IEEE 802.11n or IEEE 802.16.1
(ZigBee) and the like. In general, any description in this document
which mentions WLAN signals is applicable to narrow band signals
(compared to UWB) or to any of the IEEE WLAN standards as IEEE
802.11a/b/g but also to other standards as IEEE 802.11n, IEEE
802.16.1.
The method can be implemented in tags as well as in any standard
wireless client operating in such networks. For the sake of
simplicity, any reference to tags in this document, applies also to
mobile units or standard clients and vice versa. In addition any
reference in this document to a location transceiver is fully
applicable to an access point having the capability to measure the
TOA (Time of Arrival) of a received message. In respect to receiver
functions, a reference to location transceivers is also applicable
to location receivers (units which have no transmitters) which can
be part of a location system. In another embodiment, the location
transceiver may be attached and/or integrated and/or be a part of
the access point.
Referring now to the figures and in particular to FIG. 1, a WLAN
network 1 within which the present invention is embodied is
depicted in a pictorial diagram. Fixed-position location
transceivers 2-5 within the wireless network 1 are associated in a
group, although not necessarily all fixed-position location
transceivers 2-5 within wireless network 1 will be assigned to any
group.
According to one embodiment of the network 1 as depicted in FIG. 1,
it consists of N location transceivers 2-5 (N=2 or more) which are
wirelessly synchronized by a sync unit 6. All the location
transceivers 2-5 have data communication (wired or wireless) with a
server 15 operatively connected to the location transceivers
2-5.
Sync unit 6 includes a transmitter able to transmit WLAN messages
and/or UWB messages. In different embodiments of the present
invention, the sync unit 6 maybe one of the location transceivers
2-5, a tag 12-14, or a combination of both. In other embodiments,
more than one sync unit 6 can be used to synchronize the location
transceivers 2-5.
According to another location system embodiment, the time
synchronization between the Location transceivers is achieved by a
wired distribution of a common timing signal.
In essence, time synchronization of the location transceivers 2-5
may be performed as described in prior art publications. The
present invention focuses on the advantages of combining WLAN (e.g.
Wi-Fi) and UWB technologies for time synchronization and for
location estimation.
According to the embodiment as depicted in FIG. 1, the sync unit 6
as well as one or more tags 12-14 are in wireless range with the
location transceivers 2-5, and transmit either a WLAN signal alone
or a combination of WLAN and UWB signals 16 as will be described
below.
The transmitted WLAN and UWB signals are tied together and have a
known time relationship between them and considered to be
transmitted synchronously for the purpose of TOA calculation in a
receiver, receiving both signals.
According to one embodiment of the present invention, the location
transceivers 2-5 are able to receive the WLAN signals transmitted
by a tag 12-14 or the sync unit 6, while part or all of the
location transceivers 2-5 can also receive the UWB signals when
those are transmitted in conjunction with the WLAN signal.
Therefore, location transceivers 2-5 that are able to receive WLAN
signals only, will measure the TOA 7-10 of those WLAN signals,
while other location transceivers 2-5 that are able to receive both
WLAN and UWB signals can measure and report to the server 15 the
TOA 7-10 of either one of the signals or both signals in case both
are received. In another embodiment of the present invention, the
location transceivers 2-5 will select and report the earliest TOA
value.
Since there is a known time relationship between the transmitted
WLAN and UWB signals (both signals are synchronized) by the tag
12-14 or the sync unit 6, the server 15 can use either one of the
reported TOAs (calculated from either the WLAN or UWB signal) to
perform TDOA calculations. This means that the TDOA calculated from
the TOA values, estimated and reported by two different location
transceivers 2-5, both receiving a specific combined WLAN+UWB
transmission, and where one of the location transceivers 2-5
measured the WLAN signal and another location transceivers 2-5
measured the UWB signal, is a valid TDOA since both TOA were
estimated from synchronized signals having a known time
relationship as described above. This unique and novel combination
of both communication technologies enables the location system 1 to
achieve very significant and exclusive advantages as following
described.
Still referring to FIG. 1, the server 15 can perform time
synchronization between all the location transceivers 2-5 in a
group by using TOA values that were estimated from either a WLAN or
UWB signals. Since TOA values estimated from UWB signals have
typically much better accuracy and reliability, the server 15 can
assign a different quality factor to each location transceiver 2-5
and take this in account in the location calculation process. In a
similar way, a tag 12-14 transmitting a combined signal maybe
located from TOA values estimated from UWB or WLAN signals. This
hybrid type of location has a huge advantage in respect to the
short ranges of the UWB signals. For example, a pure UWB TOA-based
location system cannot locate a tag with less than three receivers
and in many cases four receivers are needed to achieve a good
location. This minimum number of receivers dictates a minimum
receiver density that shall be deployed in order to have location
everywhere. Since the UWB range is shorter than the WLAN range, the
receiver density is higher. Using the system 1 as described in this
invention, a location can be made even by having only one or two
TOA values estimated from UWB signals. The rest of the TOA values
can be from WLAN signals.
Although this hybrid location can suffer from some accuracy
degradation compared to a pure UWB location, it is much better than
a pure WLAN location and it doesn't require a higher density in the
location transceiver 2-5 deployment. Moreover, the server 15 can
assign a better quality factor to TOA values estimated from UWB
signals and take it in account as a weighing factor in the location
process. In one embodiment, the server 15 can be programmed for a
minimum number of UWB-based and/or WLAN-based TOA values to
calculate and report a valid location. In another embodiment, the
server 15 can report (e.g. via APIs) a quality factor of the
location based on the number of UWB-based and/or WLAN-based TOA
values used for the location process. This quality factor can also
combine the synchronization quality of each of the
transceivers.
For example, in a preferred embodiment of the system 1 the quality
of a TOA used for location can be rated as follows:
TABLE-US-00001 Receiver Quality level synchronization Received tag
signal 1 - Highest UWB UWB 2 WLAN UWB 3 UWB WLAN 4 - Lowest WLAN
WLAN
There are several ways of using this quality factor in the location
process (e.g. co-variance matrix, weigh factor during best fit
process, etc.) but a detailed description is beyond the scope of
the present invention.
Combined UWB and WLAN signals maybe used in additional ways to
improve the performance of the location system 1. In a preferred
embodiment, floor and cell differentiation can be performed based
on the UWB signals. Since these signals are strongly attenuated by
concrete floors, the server 15 can use them as an indication of tag
presence in a specific floor.
In another implementation of the system 1 according to the present
invention, the time synchronization between all the location
transceivers 2-5 in a group can be done differently from the
description above. The location transceivers 2-5 can be
synchronized using WLAN signals and from time to time (e.g. every
few seconds or minutes), UWB signals are used to detect
synchronization offsets and compensate them. This is a significant
advantage in cases where strong multipaths cause high offsets in
WLAN synchronization. This method is applicable even when the UWB
and WLAN signals are not tied together and transmitted
synchronously. The server 15 will basically synchronize the clocks
of the location transceivers 2-5 using the UWB signals but maintain
short time synchronization (few seconds or minutes) using WLAN
signals.
In another embodiment, the location transceivers 2-5 receiving both
UWB and WLAN signals can calculate by itself the offset of the WLAN
signal (based on the UWB signal) and report a corrected value. In
other cases it can decide which signal has better quality and
report the TOA based on the best signal quality. The same offset
can be used by the location transceiver 2-5 to learn about the
presence and timing of multipaths in the WLAN signal. Although
those are two signals with different characteristics, having a good
estimate of the expected TOA of the WLAN signal it is possible to
learn more about the channel characteristics and use this
information when UWB signals are not received.
Now referring to FIG. 2, an embodiment of the combined tag message
of the tag 12-14 is depicted. The WLAN message 18 has a known
format including a preamble, header and data fields. In a typical
location system, the data fields will carry tag identification as
well as telemetry data. According to an embodiment of the present
invention, the UWB signal 19 consists of a sequence of UWB pulses
carrying no data (i.e. the UWB receiver receiving this signal is
not requested to decode any data from this signal). Since both
signals are transmitted by the same tag 12-14, there is no need to
transmit data in both the UWB 19 and WLAN 18 signals. In this
embodiment, all the tag data is included in the WLAN message only.
This is a major advantage in respect to the effective range of the
UWB signal. In a pure UWB Location system, each UWB signal carries
the tag identification which requires from the UWB receiver to
receive the message with no errors (since normally only CRC codes
are used to protect the information). In that case, the effective
range of the UWB signal is limited by the energy of a single bit.
As previously mentioned and according to this preferred embodiment,
the UWB signal is only used for TOA estimation. This means that the
UWB receiver can improve the SNR of the received signal by
integrating part or all of the received UWB pulses. The range
limitations imposed by the E.sub.b/N.sub.o of a single pulse are
now considerably improved.
Also according to this embodiment, the UWB signal 19 is transmitted
after some of the WLAN fields have been transmitted (e.g. after the
message header). A combined UWB+WLAN location transceiver 2-5 can
take advantage of this scheme to start receiving the UWB signal at
a given and well specified time window. This is another advantage
of this combined transmission, since a pure UWB receiver shall set
a detection threshold high enough to avoid excessive false alarms
(due to in-band noise). In this case, the UWB receiver sensitivity
can be improved by starting the UWB detection at a well defined
time given by the received WLAN signal. Moreover, the WLAN message
header may include an indication (e.g. UWB flag) indicating whether
or not an UWB signal is also being transmitted.
In other embodiments, the UWB pulse train can include data and/or
be modulated using a spread spectrum direct sequence. Using spread
spectrum modulation is especially useful in WLAN systems working in
different channels. In this case, two tags transmitting in two
different WLAN channels may overlap in time and therefore also the
UWB signals will overlap thus creating a potential identification
problem (of the UWB signal) in the receiver. Modulating the UWB
signal with orthogonal sequences (one for each WLAN channel) will
allow the receiver to easily differentiate between two overlapping
UWB sequences. Among other well known techniques, Pulse Position
Modulation can also be used to differentiate between two or more
overlapping UWB sequences, which can also be related to the timing
of the WLAN signal. Using direct sequence modulation is also
beneficial for obtaining a better SNR of the received signal. In
other cases, the UWB pulse train can be transmitted at the
beginning of the WLAN signal or even without any overlap to it
(before or after it).
Another major advantage of this combined transmission is achieved
by the CCA (Clear channel assessment) process done for the WLAN
signal. Therefore, the UWB transmitted signal is more likely to be
clean (although CCA does not guarantee it by 100%) from
interference or overlap of other units transmitting also combined
UWB+WLAN signals. Since typical UWB systems do not use any CCA
mechanism (pure Aloha), the channel usage (directly related to the
system capacity) is limited to around 18%. With the combined
transmission, the system operates in a slotted-Aloha mode thus
improving the channel usage to around 36%. In other cases, this CCA
mechanism can be used to avoid UWB interference to other
communication systems as requested by some regulatory
organizations. Other advantages from this combined UWB+WLAN tag
transmission will be clear from the description below.
Referring now to FIG. 3, a detailed timing of a combined UWB+WLAN
signal 50 is depicted. According to this embodiment, the UWB pulse
52 is transmitted at the rising edge of the WLAN signal symbol
clock 51. Consider an IEEE 802.11b WLAN signal, using a 1 Mbps
symbol clock, the UWB pulse 52 (which may consist of one or few UWB
carrier cycles). In this case, the UWB pulses 52 will be
transmitted at a rate of 1 Mbps (the WLAN signal symbol rate).
Having a known time relationship between the WLAN and UWB signals
is beneficial in many aspects.
The main one as previously explained, is the possibility to
calculate the TDOA of a combined signal from two synchronized
location transceivers 2-5, one of the location transceivers 2-5
estimating the TOA of the UWB signal and the other location
transceivers 2-5 estimating the TOA of the WLAN signal. Since in
the vast majority of the cases, the WLAN signal will be received
with a better SNR than the UWB signal (due to the higher
transmitted power), a combined location transceivers 2-5 can use
the symbol clock of the received WLAN signal as the approximate
timing of the UWB signal. In some extreme cases, and due to large
multipaths, this timing can be significantly distorted but in most
of the cases it can define a suitable time window (e.g. .+-.100-200
nsec) for the UWB reception. By reducing the search window of the
location transceivers 2-5 receiving the UWB signal, one can improve
its sensitivity. For example, when operating in a noisy channel,
there is a higher probability to miss the detection of a UWB pulse
with a noise peak. By reducing the time slot, the location
transceivers 2-5 receiving the UWB signal is less likely to have
such a missed detection and more likely to measure the time of the
right pulse. This increased sensitivity of the location
transceivers 2-5 receiving the UWB signal can contribute to an
increased UWB effective reception range, which will increase the
area in which a specific tag 12-14 is located thus providing an
advantage over a pure UWB location system. This time window can be
programmable or automatically adjusted by the location transceivers
2-5 themselves.
Since in some preferred embodiments, the UWB pulse train includes
no data (e.g. a train of all "ones"), the TOA of the UWB signal
will have a time ambiguity of the UWB pulse interval (e.g. 1
.mu.sec in the example above). This ambiguity can be resolved using
the WLAN signal as a reference but also by assuming that the UWB
signal cannot be received at distances of more than half of the
distance the signal propagates in one time interval (e.g. in this
case .about.150 m). In case this distance is too short, the spacing
between the UWB pulses can be increased (e.g. 2 .mu.sec). Many
other well known techniques can be used to solve this ambiguity
problem.
In other implementations, the timing of the UWB signal can be
related to the chip clock or to any other time reference in the
transmitted WLAN signal. When related to a chip clock (e.g. an 11
MHz clock in an IEEE 802.11b WLAN system), a non-coherent location
transceivers 2-5 will be able to integrate many UWB pulses without
significant losses due to the difference between the transmitter
and receiver clock. In this case, ambiguity problems (as described
above) can be solved by modulating the UWB signal with a Direct
Sequence or other methods.
The transmitted UWB signal 52 can be modulated in many ways and
using any modulation with UWB bandwidth. The modulation type will
normally be selected according to the system requirements. Typical
examples include Pulse Amplitude Modulation (PAM), ON-OFF Keying
(OOK), Bi-phase Shift Keying (BPSK) and Pulse Position Modulation
(PPM), but more sophisticated modulations as SPSP-DS or multiband
OFDM can also be used.
Referring now to FIG. 4, a block diagram of a combined UWB+WLAN tag
20 is depicted. The tag 20 may be used in the system 1 as one or
more of the tags 12-14. In this embodiment, the tag 20 consists of
a Tag controller 21, a WLAN transmitter 23, WLAN receiver 24, an
UWB transmitter 22, a WLAN power amplifier (PA) 26, a Low Noise
Amplifier (LNA) 25, a T/R switch 28 and a WLAN antenna 29. The UWB
signal is transmitted using a separate antenna 27. The tag
controller 21 controls the whole operation of the tag 20 including
message preparation, transmitter and receiver control and any other
task required to ensure proper tag operation. The UWB transmitter
22 and WLAN transmitter 23 are interconnected to provide common
timing required during transmission.
In a preferred embodiment, the UWB transmission is performed at a
carrier frequency of 6.5 GHz and a bandwidth of 500 MHz. The
transmission power is according to the FCC regulations as it does
not exceed the emission limits of -41.3 dBm/MHz.
In other preferred embodiments the UWB signal is transmitted with
an UWB carrier frequency in the range of 3.1-10.6 GHz and a
bandwidth equal or greater than 500 MHz.
According to one embodiment of the present invention, a typical
combined UWB+WLAN location system will include tags transmitting
combined UWB+WLAN signals but which can only receive WLAN
signals.
The WLAN transmitter 23 can be used to transmit any kind of tag
identification as well as other telemetry data such as tag status,
sensor data, external host data, etc. The WLAN transceiver can be
used to send any required information to the tag including
programmable parameters, download firmware, etc. Having 2-way
communication is beneficial for secure and reliable communication
requiring Ack/Nack mechanism, encryption and authentication. This
is another main advantage of this combined system 1 according to
the present invention. A pure 2-way UWB tag requires the
implementation of a UWB receiver in the tag which significantly
complicates it and increases its cost. Since there is no need for
an additional data downlink (from the system to the tag), the UWB
receiver in the tag can be avoided.
In other embodiments of the tag 20, where only 1-way communication
(from the tag to the system) is required, it is also possible to
remove the WLAN receiver 24 too or leave just an energy detector
for CCA purposes.
Referring now to the tag antennas 27 and 29, those units can be
implemented as a single dual-band antenna with a single feed point
(UWB+WLAN) or as two antennas assembled together with two separates
feed points.
Referring now to FIG. 5 below, one embodiment of a location
transceiver 30 is depicted. The location transceiver 30 may used in
the system 1 as one or more of the location transceivers 2-5. The
location transceiver 30 typically consists of a controller 31, a
WLAN transmitter 37 and WLAN receiver 38, a WLAN signal power
amplifier 43, a WLAN signal low noise amplifier 44, a T/R switch 45
and WLAN signal antenna 32, an UWB receiver 36 with UWB antenna 41
and timing function 40. Although not shown in the block diagram,
the timing function 40 interfaces with most of the functions in the
transceiver unit. The timing function 40 includes an internal time
source (e.g. TCXO or OCXO) and the related hardware (e.g. PLL,
dividers, counters, etc.) to provide the required timing signals to
other functions in the transceiver. Thus, this timing function 40
provides the source clock to generate the RF frequency in the WLAN
transmitter 37 and WLAN receiver 38, the timing and clock for the
UWB receiver 36, the sampling clock for the A/D 34, the timing of
the Matched Filter and RAM 32 (e.g. address counter to store
samples) and the TOA counting for the UWB TOA estimate function 46
and for the WLAN TOA estimate function performed by the controller
31. The location transceiver 30 further includes circuitry for the
WLAN signal TOA estimation which includes an A/D converter 34 for
sampling of the I&Q data 35A and 35B, a multiplexer 39 to
select the I&Q signals to be sampled and a matched filter and
RAM 32 to process and store the sampled signals 33 and circuitry to
estimate the TOA of the received UWB signal 46. For the sake of
simplicity external interfaces (e.g. power, Ethernet, etc.) are not
shown. Also the use of diversity antennas is obvious and not
depicted. According to this embodiment, the location transceiver 30
can handle both UWB and WLAN signals when those are transmitted
together by a combined UWB+WLAN tag or when transmitted separately
by two different tags.
The TOA estimation of a WLAN signal is performed by sampling the
received I&Q 35B baseband signal and by using a matched filter
to find the coarse timing of the received signal. The fine TOA of
the WLAN signal is then estimated by the controller using signal
processing techniques well known to the skilled in the art and
beyond the scope of this invention. Sampling I&Q transmitted
signals 35A allows the location transceiver 30 to accurately
estimate the TOA of the transmitted WLAN signal and perform self
synchronization functions as described in prior art
publications.
The TOA estimate of the UWB signal can be performed by threshold
detector or using more sophisticated well known techniques. As
previously mentioned, the TOA can be estimated on an integrated UWB
signal (the integration can be coherent or non-coherent and can be
done inside the UWB TOA estimate block 46.
In another embodiment, the timing of the UWB receiver 36 can be
synchronized 47 to the data received in the WLAN receiver 38. The
UWB receiver 36 will detect the UWB signal in narrow time windows
synchronized to the WLAN received data 35B.
The shown location transceiver 30 configuration as well as the use
of I&Q signals for TOA estimation is just one of possible
preferred embodiments. It's also possible to use low IF sampling or
partial processing in IF and achieve the same functionality. To the
skilled in the art, it should be obvious that different embodiments
may include either two separate TOA estimate functions or a unified
and combined function. In either one of those cases, both TOA
estimate functions (separate or combined) use a common timing
function 40 and therefore the estimated TOA values are both
referenced to synchronized time counters.
In accordance with one embodiment, the timing function 40 includes
two TOA counters used for time stamping of each of the received
signals and those two TOA counters run synchronously from a common
timing signal. Although those counters may have a different time
resolution, they preferably have the same overall time cycle or
alternatively times cycles which have a ratio between them equal to
an integer n (where n=2.sup.k, k=0, 1, 2, 3 . . . ). In another
embodiment, the TOA counter is the same for both TOA estimate
functions (UWB and WLAN).
In accordance with another embodiment, the timing function 40 is
connected to an external timing signal 48 provided by a central
clock source which provides this common timing signal to a
plurality of transceivers through a wired interface. In that case,
this plurality of transceivers operates with a synchronized time
base generated by this common timing signal.
Another implementation may include two controllers 31, a master
controlling the WLAN communication activity and a slave controlling
the UWB reception activity. When the location transceiver 30
receives both UWB and WLAN signals, the location transceiver 30 can
report both TOA values to the server 15 (FIG. 1) (both TOA values
being referenced to a common time counter or to synchronized time
counters) or just one according to a quality test performed by the
transceiver controller. In addition, the controller can use the
UWB-based TOA to learn the channel characteristics of the WLAN
signal. Those channel characteristics (e.g. delay spread and
multipaths) can be used by the transceiver controller 31 to better
estimate the WLAN signal TOA when the UWB signal is not
present.
In another embodiment, the location transceiver 30 can always
report the estimated TOA of the WLAN signal but correct it
according to the TOA of an UWB signal when such a signal is also
received. That way the location server 15 deals with only one
reported TOA from each location transceiver 30, those reports
including a quality factor based on the received signals.
In another embodiment, a location transceiver 30 can receive and
report the TOA of an UWB signal although the WLAN signal was not
received (e.g. due to CRC error). Even in those cases where the UWB
signal carries no data, it is possible to identify this signal
based on other transceivers that received the WLAN signal (with the
tag identification) and with a TOA very close to the TOA of the UWB
signal received by the first transceiver. This identification is
performed in the server 15 and allows using UWB signals for
location even when the WLAN signal was properly received in at
least one location transceiver.
Other embodiments of the location transceiver 30 may include the
integration of a dual UWB receiver 36 which can be used for
accurate Angle of Arrival (AOA) estimation of the UWB signal. A
location server 15 using both TOA and AOA information will normally
be able to calculate a more accurate location or even calculate a
good location with only two location transceivers 30.
Now referring to FIG. 6, the TOA counters section 60 comprised in
the timing function 40 (FIG. 5) of the location transceiver 30
(FIG. 5) is depicted. According to one preferred embodiment, this
section includes two separate counters 70 and 71 for time stamping
the TOA of the UWB 72 and WLAN 73 signals respectively. Those two
counters may have different length and count at different
frequencies f.sub.WLAN 67 and f.sub.UWB 68 but preferably those two
frequencies have a ratio between them equal to an integer n (where
n=2 k, k=0, 1, 2, 3 . . . ) since this significantly simplifies the
TDOA calculations. To start counting from a known point, the TOA
counters 70 and 71 are initialized from a common reset signal
69.
The clock source used to generate the counting frequencies
f.sub.WLAN 67 and f.sub.UWB 68 is preferably provided by a good and
stable crystal source (e.g. TCXO or OCXO) 61 but in other cases it
can be externally provided by an external timing signal 62 commonly
distributed to a plurality of transceivers by a central clock
source. The controller 31 (FIG. 5) can select 64 which clock source
is used. Also according to this preferred embodiment, the source
clock is multiplied by a PLL 65 in order to achieve a higher
counting frequency (e.g. 44-200 MHz) as required by the TOA
functions. Typically the UWB TOA counter 70 will use a higher
counting frequency due to the higher TOA resolution 72 used by the
UWB TOA estimate function 46 (FIG. 5). For that reason a divider 66
is used to accordingly reduce the PLL 65 output frequency to the
counting frequency 67 of the WLAN TOA counter 71.
In another preferred embodiment, both TOA counters 70 and 71 are
implemented as a single counter, from which each TOA estimate
function uses a specific section of this TOA counter.
The proposed dual bandwidth TDOA system 1 has many advantages as
were described above. The proposed system leverages the advantages
of either one of the two (WLAN and UWB) location systems and
creates a combined system with an overall better performance than
any separate system. Those additional capabilities include both an
upgrade of the WLAN TDOA location system accuracy as well as an
upgrade of the UWB location system coverage.
This disclosure provides exemplary embodiments of the present
invention. The scope of the present invention is not limited by
these exemplary embodiments. Numerous variations, whether
explicitly provided for by the specification or implied by the
specification, such as variations in structure, dimension, type of
material and manufacturing process, may be implemented by one
skilled in the art in view of this disclosure.
* * * * *